A various kinds of measures such as the installation of a flow resistor such as a perforated panel, a baffle plate, etc., the installation of a large number of inlet ports, the installation of a large volume of suction space under the floor, etc. have been taken in order to achieve a uniform distribution of sucked fluid velocity at an inlet port.
Those conventional measures can not actually achieve uniform distribution of sucked fluid velocity and have problems such as increase of pressure loss, increase of equipment cost, etc.